Previous work from this laboratory (J. Gen. Physiol. 170: 55, 1970) has shown that solutes passing through a membrane interact in a fashion not before described. As a result of the interaction of a driver solute, for which there is a concentration gradient, and a tracer solute with no concentration gradient, there is developed an assymmetric flux of the tracer solute. This phenomenon has been called solute drag. Having established solute drag as a process common to transmembrane diffusion of both biological and synthetic membrane, we now propose to test the mechanism of solute drag in naturally occurring membrane processes. The work will include solute drag studies on both in vivo and in vitro membrane systems. Intestine and bladder preparations will be studied in vitro and sheep placenta exchange studies will constitute the first in vivo preparations. Maternal-fetal exchange will be monitored with solutions perfused through umbilical arteries and veins. Tracers with tritium or C14 labels will be perfused and/or collected in both directions across the placenta. Assymmetric tracer fluxes in the presence of concentration gradients of permeable hyperosmolar solutes will indicate the presence of solute drag.